1. Field of the Invention
This invention relates generally to marine seismic surveying, and, more particularly, to water velocity decomposition in marine seismic surveying.
2. Description of the Related Art
Seismic exploration is widely used to locate and/or survey subterranean geological formations for hydrocarbon deposits. Since many commercially valuable hydrocarbon deposits are located beneath bodies of water, various types of marine seismic surveys have been developed. In a typical marine seismic survey, such as the exemplary survey conceptually illustrated in FIG. 1, an array 100 of marine seismic streamer cables 105 is towed behind a survey vessel 110 over a survey area 115. The seismic streamer cables 105 may be several thousand meters long and contain a large number of sensors 125, such as hydrophones and associated electronic equipment, which are distributed along the length of the each seismic streamer cable 105. The survey vessel 110 also tows one or more seismic sources 120, such as airguns and the like.
As the array 100 is towed over the survey area 115, acoustic signals, or “shots,” produced by the seismic sources 120 are directed down through the water into the earth beneath (not shown), where they are reflected from the various subterranean geological formations. The reflected signals are received by the sensors 125 in the seismic streamer cables 105, digitized and then transmitted to the survey vessel 110. The digitized signals are referred to as “traces” and are recorded and at least partially processed at the survey vessel 110. The ultimate aim of this process is to build up a representation of the subterranean geological formations beneath the array 100. Analysis of the representation may indicate probable locations of hydrocarbon deposits in the subterranean geological formations.
Since the area of the array 100 is typically much smaller than the survey area 115, a representation of the earth strata in the survey area 115 may be formed by combining data collected along a plurality of sail lines 130(1-n). For example, a single survey vessel 110 may tow a single array 100 along each of the sail lines 130(1-n). Alternatively, a plurality of survey vessels 110 may tow a plurality of arrays 100 along a corresponding plurality of the sail lines 130(1-n). However, variations in the water conditions, e.g. water temperature, salinity, and the like, between the plurality of sail lines 130(1-n) may cause variations in the velocity of sound in water among the sail lines 130(1-n). For example, the variations in seismic travel time can be on the order of 10 or 20 milliseconds for traces having a small distance between the source and detector for surveys carried out in deeper waters (greater than 200 m). The variations in the seismic wave travel times may shift the temporal position of the various events recorded in the seismic data, including, but not limited to, reflections and refractions of the seismic waves from the subterranean geological formations beneath the array 100. Consequently, the variations in the travel times may make it difficult to analyze the combined seismic data set and may reduce the accuracy of the survey.
Moreover, the data for the sail lines 130(1-n) may be collected at different times. For one example, a single pass along one of the sail lines 130(1-n) may take several hours to complete so, if a single survey vessel 110 is used, data for the first sail line 130(1) will be recorded at an earlier time than data for the last sail line 130(n). For another example, inclement weather and/or high seas may force a survey to be suspended before resuming hours or days later. For yet another example, historical data from previous surveys performed months or years earlier may be combined with new data to extend the survey or to fill in deficiencies in coverage that may be introduced by currents, obstacles such as platforms, and the like. And for yet another example, data from repeat surveys may be used to analyze and monitor changes in productive oil and/or gas reservoirs.
Combining data from different times, and especially from different surveys, may exacerbate the aforementioned difficulties associated with variations in the velocity of sound in the water layer. For example, seasonal variations of the water temperature, salinity, and the like, may cause pronounced variations in the velocity of sound in water. For another example, shifts in water currents may cause unpredictable variations in the velocity of sound in water, particularly for surveys carried out near the edge of strong water currents.
The seismic data is commonly corrected for the variations in the velocity of sound in water by computing one or more so-called “delta t” (Δt) values, which are typically defined as a difference between an expected travel time, usually based on an assumed ideal water velocity, and a measured travel time for one or more seismic signals. For example, the assumed ideal water velocity may be a constant velocity or one with very smooth spatial changes in velocity.
In one conventional method of determining the delta-t values (described in Wombell, R., 1996, “Water velocity variations in 3-D seismic processing,” 66th Ann. Internat. Mtg: Society of Exploration Geophysicists, Expanded Abstracts, 1666-1669.), normal move-out stacking velocities and zero-offset water bottom times are computed along adjacent sail lines. The velocities are then converted to zero offset travel time differences using the formula: Δt=Tw(ΔVw/Vw), where Δt is the difference in two-way travel time at zero offset due to the change in water velocity, Tw is the zero offset water bottom time, Vw is the reference water velocity chosen by the practitioner, and ΔVw is the difference between Vw and the computed stacking velocity. The delta-t values are then applied to normal move-out corrected seismic data. One problem with this method is that the velocity analysis must be extremely accurate. Another issue is the effect of water bottom structure on the velocity analysis. If the dip of the water bottom (the angle the water bottom makes with a horizontal plane) changes between or along sail lines, the calculated velocities are strongly affected and may reduce the accuracy of the Δt calculation.
Another method of determining the delta-t values that may be used to form combined data sets is described in Fried, J., and MacKay, S., 2001, “Dynamic Corrections for Water Velocity Variations: a Nova Scotia case history,” Canadian Society of Exploration Geophysicists, October 2001 technical luncheon. In this method, normal move-out corrections are applied to pick times with a single velocity. The corrected pick times are then averaged for each combination of sail line, cross line, and common midpoint. The differences between sail line-cross line groups having overlapping midpoints are then evaluated and reduced using an iterative method. The method produces delta-t values that are used to apply a dynamic correction to the seismic data. One difficulty with this method is that the averaged pick times are affected by the difference between the actual normal move-out and the approximate normal move-out applied to correct the pick times. The effect of these differences propagates into the delta-t values. Reducing the offset range of pick times used in the average reduces the differences between the actual normal move-out and the approximate normal move-out applied to correct the pick times. However, reducing the offset range may also reduce the amount of overlapping data upon which the method depends. Also, since the move-out is affected by the dip of the water bottom, changes in dip between sail lines can also affect the delta-t values. Furthermore, the iteration procedures used in this method are difficult to apply in practice.